Cell-cell Interactions During Pollination:

Fertilization in plants is remarkably species-specific; a series of cell-cell interactions enables female cells to selectively interact with appropriate pollen grains. During the past few years, our laboratory has identified many genes that control communication between pollen and the pistil cells. We have discovered mutations that define key stages in fertilization, from the adhesion of pollen grains to female cells (Zinkl et al., Development, 126: 5431-5440) to the targeting of pollen tubes to eggs — a process strikingly similar to axon guidance in animals (Wilhelmi and Preuss, 1996, Science 274: 1535-37). We are finding that these cell-cell interactions employ molecules that are quite different from those analyzed before, relying more often on lipohilic components than on protein-protein binding. We recently demonstrated that pollen adhesion depends on a species-specific lipophilic adhesive that generates a remarkably strong binding force between pollen grains and female cells (Zinkl et al., Development, 126: 5431-5440). This investigation has had an unexpected benefit — further analysis of the "molecular velcro" that mediates pollen adhesion promises to yield an array of molecules that bind in a dry environment in a selective manner.

Other hydrophobic molecules embedded in a lipophilic extracellular matrix mediate pollen recognition (Preuss et al., 1993, Genes Dev. 7: 974-985), and lipids are necessary for the assembly and function of hydrophobic proteins on the pollen surface (Mayfield and Preuss, Nature Cell Biology 2: 128-130). Our work suggests that this array of lipids and proteins together establish the initial interactions that support pollen-pistil signaling; we are currently testing this hypothesis. Even pollen tube guidance may depend on lipophilic cues — with the recent cloning of a gene that mediates pollen tube targeting (Wilhelmi and Preuss, in preparation) we have demonstrated an interesting connection to the biotinylated enzymes that regulate synthesis of hydrophobic signaling molecules.

Future objectives:
Over the next few years, we will continue to explore the genes and pathways that allow plants to selectively interact with appropriate pollen. Our current work is focused on identifying the molecules that enable pollen to bind to female cells of an appropriate species, and unraveling the signals that promote proper guidance of pollen tubes to the ovules. At the same time, we have developed methods that extract and purify the surface proteins from pollen. The availability of a robust genetic system provides a considerable advantage for investigating these questions. Purification strategies and biochemical assays benefit from the available array of mutants, and reverse genetic approaches are effective at pinpointing the function of individual protein components. Ultimately, these methods will allow us to unravel the code, whether specified by proteins or other molecules, that identifies each pollen grain as belonging to a particular species.